A similar thermal overload protection device of the above mentioned species is known in the art from German patent DE 39 21 225 C1. Therein, the electronic component is a gas-filled, three-pole surge arrester. The central electrode is pressed against the melt element under the action of the spring-elastic shorting link. The shorting link comprises two laterally projecting contact fingers, being held spaced to the outside electrodes of the surge arrester. For an overload existing for a longer period of time, the melt element will fuse, so that the shorting link connects, with its two contact fingers, the two outside electrodes directly with the earthed central electrode, thus the surge arrester being protected against damage.
With the prior art thermal overload protection device for electronic components as a so-called FAIL SAFE protection device, it is possible, in particular for common gas-filled surge arresters, to reduce the risk of overheating. This is achieved by the melt element, which may be a solder pill or another thermally sensitive element and which fuses after a limit temperature at the surface of the surge arrester has been exceeded and yields to the spring-loaded shorting link. It has shown, however, that an arbitrarily high spring force cannot be selected for such a thermal overload protection device, since otherwise, a plastic deformation of the melt element may occur in the operating temperature range and may lead to an undesired shorting of the electrodes of the surge arrester. With the common spring forces, the obtainable contact forces between the shorting link and the electrodes are, however, too small, so that a surge-current resistant shorting cannot be achieved, the rated arrester surge current being regarded as the surge current. In the most unfavorable case, when the FAIL SAFE device is tripped by an a.c. load of the surge arrester, and a surge load is caused, the contact finger of the shorting link may be damaged, thus the FAIL SAFE device being made ineffective, and an overheating of the surge arrester being possibly generated, which may even lead to a fire.
Electronic components may be provided with a thermal overload protection device such as the gas-filled surge arresters in two or three-pole designs. Also semiconductor surge arrester devices can be employed, such as triacs, thyristors, Zener diodes and the like.